The present invention relates to a device for solidly connecting a shell of a prosthetic acetabular cup to the acetabular bone mass. The device has at least one attachment element taking the form of a threaded rod or screw passing through at least one aperture or hole in the acetabular cup shell. The aperture in the shell includes a ring mounted therein such that it can lock the angular position of the screw in the shell after it is screwed into the bone. The shell can be in the form of a typical spherical acetabular cup shell for contacting the natural acetabulum.
Known screw-and-plate osteosynthesis systems must allow immobilizing one or more bone fragments in reference to others. It is known to use spherical head screws cooperating with a spherically-shaped aperture in a plate and bringing the plate into compression over the bone until the friction of the plate on the bone stabilizes the assembly. These assemblies allow choice of the angle of implantation of screws during the operation and cause a return movement and a compression of a detached bone fragment. Certain of these systems allow, due to the oblong shape of the aperture, made in the plate, compression of one bone fragment on another.
Use is also known of a second generation of screw-and-plate systems called monoaxial-locking and polyaxial-locking systems in which the strength of the assembly no longer depends on compression of the plate on the bone but on a fixation of the screw in the plate. These systems allow achieving assembly away from the bone with, for the more elaborate ones, the possibility of choosing the angle of implantation of screws during the operation while achieving strength sufficient for postoperative stresses.
One example of monoaxial-locking device is described in French patent publication FR-A-2,739,151. This device comprises a plate traversed by tapped tapered holes in which are wedged the tapered threaded heads of the screws after tightening. U.S. application Ser. No. 10/999,132 teaches a method of locking the polyaxial screw, the teachings of U.S. application Ser. No. 10/999,132 are incorporated herein by reference.
In existing monoaxial mechanisms, bone anchoring is sometimes improved by the introduction of screws with the axes between them not being parallel. To achieve such orientation of the screws, the plate to be fixed to the support can be provided with holes with axis non-orthogonal to the surface of the plate such that the orientation of the screw is provided by the orientation of the hole in the plate. The surgeon then has no freedom concerning the orientation of the screw to be fixed. This predetermined orientation may sometimes prove to be incompatible with anatomical constraints.
One example of a polyaxial locking mechanism is specifically described in French patent publication FR-A-2,790,198. This device comprises a plate equipped with a ring expanding radially during the threading of the screw to allow immobilization of the ring and then of the screw inside the hole of the plate. The expansion of the ring is obtained by a tapered threading of the screw and of the internal bore of the ring. Another polyaxial system is shown in U.S. Pat. No. 5,954,722.
A system is likewise known through patent DE 100 39 767 comprising a ring with spherical external shape being able to be adjusted inside a plate comprising a spherically-shaped aperture. The ring is transversed by a screw, whose head is housed in a bore of the ring. No locking system is described for the screw.
U.S. Pat. Nos. 6,235,003 and 6,575,975 relate to a system comprising a ring whose outside shape is spherical, orientable inside a plate comprising a spherical housing. The ring is transvered by a screw, whose head is housed in a cylindrical bore of the ring. Locking is obtained by expansion of the head of the screw using an additional tapered screw.
U.S. Pat. No. 5,531,554 relates to a system comprising a malleable-collar screw. The collar is restricted when it is driven into a tapered hole on the plate until it spreads beyond the said hole, since it expands due to elasticity to prevent the screw from being removed. Return movement of the collar of the screw to its initial shape also inhibits axial displacement of the screw in the sense of an extraction of the screw from the bore in the plate.
WO 2004/028334 A2 shows a non-circular ring mounted in a non-circular hole.
It should be noted that other systems achieve locking by introduction under force of the threaded head of the screw in the aperture of the plate. The force necessary for the deformation of the material in the aperture is significant. Moreover, risks from detachment of metal particles cannot be ruled out.
Finally Patent Publication US-A1-2001/037112 relates to a system comprising a ring whose outside shape is spherical, orientable inside a plate comprising a spherical aperture. The ring is transversed by a screw, whose head is housed in a cylindrical bore of the ring. Locking is obtained by expansion of the head of the screw using an additional tapered screw.